The invention is particularly concerned with sensitive precision instruments of the kind in which a mass which is sufficiently great to constitute an inertial frame of reference is suspended so as to be free to move, by rotation or by translation, with respect to the instrument as a whole, so that motion of the instrument can be detected or measured. For ease of reference, such instruments as a class will be referred to herein as pendulum instruments. Pendulum instruments include accelerometers, velocimeters, displacement meters including inclinometers, gravity meters, and some kinds of inertial navigation instruments, as well as seismometers of the kind in which a mass is suspended in a frame which is intended in use to contact, and be moved by, the surface of the earth (or any other body whose seismic vibrations are to be measured). In principle, the vibrations cause the frame to move, and the mass is sufficiently freely suspended and sufficiently massive to form its own frame of reference, and the relative motion between the mass and the frame is detected and measured.
By limiting the degrees of freedom relative to the frame in which the mass can move, the seismometer can be set up to measure movement on one particular axis. This may be rectilinear motion, and this is approximated to in many pendulum-type seismometers by arcuate motion, when a long pendulum moves in a short arc.
In general, therefore, the invention is concerned with pendulum instruments in which a mass is prevented from moving in undesired directions, but is allowed to move as freely as possible in one or more other directions. The invention is specifically concerned with such instruments in which there is a mechanical linkage by which the mass is suspended and constrained with respect to the frame.
In instruments of this kind, the design of the suspension can be critical to the performance of the instrument. The suspension must be strong enough to reliably support the mass and prevent its movement in unwanted directions, yet be stable, smooth and substantially free of friction.
Given that the suspension must be resistant to movement of the mass off its intended axis, but compliant to movement of the mass in a direction in which the desired seismic disturbances are to be detected, and considering the magnitude of the mass involved and the desired sensitivity of the instrument, it is clearly desirable to provide means for locking the mass against movement with respect to the frame, so that the instrument can be transported and handled safely, without risk of damaging the delicate components. In practice, this poses problems, because it is extremely difficult to manufacture a locking mechanism that perfectly engages the mass without exerting unwanted stresses on the suspension. Indeed, it is common for suspension components to be stressed and ultimately to fracture after repeated locking and unlocking cycles.